Electrochemical machining process using current density controlling techniques

ABSTRACT

An electrochemical machining process using current density controlling techniques is disclosed In the electrochemical machining process of this invention, a carbon cathode rod activated with a negative voltage and a workpiece activated with a positive voltage are sunk into an electrolyte contained in a container, and so the workpiece is electrochemically machined while properly controlling both the metal ion dissolving rate and the metal ion diffusing rate of the workpiece by controlling the amount of applied current to maintain the two rates at a desired balance. This process thus creates a diffusion effect thickening the tip of the cylindrical workpiece, and compensates for a conventional geometric effect sharpening the tip of the workpiece. Therefore, this process produces a precise product having a uniform diameter along its length. In the electrochemical machining process of this invention, the workpiece is ultrasonically washed on its surface with both acetone and distilled water before the process so as to remove impurities from the surface of the workpiece. In addition, the electrolyte is a potassium hydroxide solution having a mole number of 4˜6 mol.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to electrochemical machining processes for removing excess metal by electrolytic dissolution, effected by a tool acting as the cathode against a workpiece acting as the anode and, more particularly, to an electrochemical machining process using current density controlling techniques, designed to electrochemically machine workpieces while controlling the amount of an applied electric current, thus effectively producing a precise product having a uniform shape in addition to precise products having a variety of shapes.

[0003] 2.Description of the Prior Art

[0004] As well known to those skilled in the art, an electrochemical machining process, also known as an electrolytic machining process, means a process, in which a workpiece in an electrolyte is electrochemically reacted in response to applied voltages to be dissolved into the electrolyte. Such an electrochemical machining process is typically carried out in four steps as follows.

[0005] That is, the conventional electrochemical machining process comprises the first step of transferring the ions of the electrolyte to the surface of an electrode, the second step of reacting the metal atoms of the surface of the workpiece with the transferred ions of the electrolyte to form particles, the third step of changing the particles into stable ions, and the fourth step of diffusing the stable ions into the electrolyte.

[0006] Such electrochemical machining processes are also classified into electrochemical polishing processes and electrochemical etching processes in accordance with results from a comparison of the processing rate of the second step with the processing rate of the third step. That is, the first processing rate when the metal atoms of the surface of the workpiece are reacted with the transferred ions of the electrolyte to form particles in the second step and the second processing rate when the particles are changed into stable ions in the third step are primarily measured prior to comparing the two processing rates with each other. When the first processing rate is higher than the second processing rate, the electrochemical machining process is an electrochemical polishing process. When the first processing rate is lower than the second processing rate, the electrochemical machining process is an electrochemical etching process. During such electrochemical machining processes, the difference between the processing rates in the above-mentioned four steps is an important factor that determines the surface conditions of the workpiece in addition to the machined shape of the workpiece. On the other hand, the metal dissolution rate in an electrochemical machining process is determined by the fourth step of diffusing the stable ions into the electrolyte.

[0007] Of the conventional electrochemical machining processes, the electrochemical etching processes are used specifically for machining micro probes having a precision of several nanometers. The electrochemical etching processes for machining such micro probes are typically performed with somewhat low concentrations of electrolytes and electric current. During an electrochemical etching process for machining a micro probe, the metal dissolution rate is higher at the tip of the probe having a large curvature than the sidewall of said probe, thus making the tip have an unwanted conical shape. Such an effect undesirable forming the conical tip during an electrochemical etching process is a so-called “geometric effect” in the art.

[0008] However, such conventional electrochemical etching processes have the following problems.

[0009] That is, the processing conditions for a workpiece during an electrochemical etching process are different in accordance with the depths of the parts of said workpiece within an electrolyte, and so the metal dissolution rate of the workpiece is partially uneven. It is thus almost impossible for the conventional electrochemical etching process to produce a precise product having a uniform shape. Another problem experienced in the conventional electrochemical etching process resides in that it is almost impossible to produce precise products having a variety of shapes due to the nonuniform metal dissolution rates.

SUMMARY OF THE INVENTION

[0010] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an electrochemical machining process using current density controlling techniques, which electrochemically machines a workpiece while controlling the amount of an applied electric current, thus effectively producing a precise product having a uniform shape.

[0011] Another object of the present invention is to provide an electrochemical machining process using current density controlling techniques, which electrochemically machines workpieces while controlling the amount of an applied electric current, thus producing precise products having a variety of shapes.

[0012] In order to accomplish the above object, the present invention provides an electrochemical machining process using current density controlling techniques, comprising: a contact point measuring step of sinking a cathode rod activated with a negative voltage into an electrolyte within a container, and feeding a cylindrical workpiece having a predetermined length and activated with a positive voltage to the surface of the electrolyte until the workpiece comes into contact with the electrolyte while measuring a contact point, at which an electric current initially flows into the electrolyte; a machining preparing step of feeding the workpiece to the surface of the electrolyte and removing the applied voltage from the workpiece, and sinking the workpiece in the electrolyte by a length, which is predetermined on the basis of the contact point and to which the workpiece has to be machined; an initial value setting step of setting a target length of the workpiece, a target diameter of the workpiece, an electrochemical equivalent volume constant of the workpiece, a current density, and machining intervals; a machining step of applying voltages to both the workpiece and the cathode rod to electrochemically machine the workpiece while continuously calculating and measuring a variable surface area of the workpiece, the amount of applied current, the amount of electricity according to the applied current, and a variable diameter of the workpiece in accordance with the lapse in machining time; and a process-end determining step of determining whether the diameter of the machined workpiece from the machining step is equal to the target diameter, thus repeating the machining step until the target diameter of the workpiece is accomplished or stopping the machining step when the target diameter of the workpiece is accomplished.

[0013] In the above-mentioned electrochemical machining process, the variable surface area of the workpiece during the machining step is calculated by the expression, A_(m)=π[LD+h(D_(o)+2D)/3], wherein A_(m) is the variable surface area (mm²) of the workpiece during machining, L is a target length (mm) of the workpiece, h is a contact length (mm) of the workpiece due to the surface tension, D is the variable diameter (mm) of the workpiece during machining, and D_(o) is an original diameter (mm) of the workpiece.

[0014] In addition, the amount of applied current during the machining step is calculated by the expression, i=A_(m)J, wherein i is the applied current (C/sec) during a unit of time, A_(m) is the variable surface area (mm²) of the workpiece during machining, and J is the current density (C/mm²sec).

[0015] The amount of electricity during the machining step is calculated by the expression, Q_(t)=Q_(p)+iΔt, wherein Q_(t) is the total amount of applied electricity (C) during machining, Q_(p) is the amount of electricity (C) applied during the previous step, and Δt is a variable machining time (sec).

[0016] In addition, the variable diameter of the workpiece during the machining step is calculated by the expression, π(D_(o)−D)[L(D_(o)+D)/4+h(3D_(o)+2D)/15]α_(e)=Q_(t), wherein D is the variable diameter (mm) of the workpiece during machining, D_(o) is the original diameter (mm) of the workpiece, Q_(t) is the total amount of applied electricity (C) during machining, L is the target length (mm) of the workpiece, h is the contact length (mm) of the workpiece due to the surface tension, and α_(e) is the electrochemical equivalent volume constant (mm³/C) of the workpiece.

[0017] In the electrochemical machining process of this invention, both the metal ion dissolving rate and the metal ion diffusing rate of the workpiece are controlled by controlling the amount of the applied current.

[0018] In addition, the cathode rod may be somewhat freely selected from a variety of conductive rods, but it is preferable to use a carbon rod as the cathode rod.

[0019] The electrolyte may be selected from a variety of conventional acid solutions or basic solutions, which have been typically used in such electrochemical machining processes. But, it is preferred to use a potassium hydroxide solution having a mole number of 4˜6 mol as the electrolyte in the machining process of this invention.

[0020] In the electrochemical machining process, the workpiece is ultrasonically washed on its surface with both acetone and distilled water before the contact point measuring step so as to remove impurities from the surface of the workpiece.

[0021] On the other hand, the additionally machined volume of metal of the workpiece due to the surface tension in the electrochemical machining process is calculated by the expression, V_(p)=πh(−2D²−D_(o)D+3D₀ ²)/15, wherein V_(p) is the additionally machined volume (mm³) of metal of the workpiece due to the surface tension, h is the contact length (mm) of the workpiece due to the surface tension, D is the variable diameter (mm) of the workpiece during machining, and D_(o) is the original diameter (mm) of the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in con-junction with the accompanying drawings, in which:

[0023]FIG. 1 is a flowchart of an electrochemical machining process using current density controlling techniques in accordance with the preferred embodiment of the present invention;

[0024]FIG. 2 is a diagram, showing a system for performing the electrochemical machining process using the current density controlling techniques in accordance with the preferred embodiment of this invention;

[0025]FIG. 3 is a flowchart, showing in detail the flow of both the machining step and the process-end determining step included in the electrochemical machining process of FIG. 1; and

[0026]FIG. 4 is a flowchart, showing in detail the flow of the contact point measuring step included in the electrochemical machining process of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0027] In the description of this invention, the technical term “electrochemical machining process” means a process, in which excess metal of a workpiece is removed by electrolytic dissolution, effected by the transferring of ions of an electrolyte to the workpiece while controlling its current density with a tool acting as the cathode against the workpiece acting as the anode. When the electrochemical machining process is performed with the workpiece brought into contact with the tool, the process is a so-called “electrochemical grinding process”. On the other hand, when the electrochemical machining process is performed with the workpiece spaced apart from the tool, the process is a so-called “electrolytic-type carving process”. When the term “electrochemical machining process” is used without specific restriction in meaning, the process is typically regarded as the electrolytic-type carving process.

[0028] When voltages are applied to both the tool acting as the cathode sunk into the electrolyte and the workpiece acting as the anode during an electrochemical machining process, electrons of the cathode acting as the anode are changed into metal ions prior to being dissolved into the electrolyte. On the other hand, the ions of the tool acting as the cathode receive the electrons, and are changed into atoms or particles prior to being deposited. That is, an oxidation occurs in the workpiece, while a reduction occurs in the tool. During the electrochemical machining process, the workpiece acting as the anode is dissolved into the electrolyte, thus electrochemically forming a desired product.

[0029] Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components.

[0030]FIG. 1 is a flowchart of an electrochemical machining process using current density controlling techniques in accordance with the preferred embodiment of the present invention. As shown in the flowchart, the electrochemical machining process of this invention comprises five steps: a contact point measuring step S10, a machining preparing step S20, an initial value setting step S30, a machining step S40, and a process-end determining step S50.

[0031] At the contact point measuring step S10, a cathode rod 1 activated with a negative voltage is sunk into an electrolyte 5 within a container. A cylindrical workpiece 3, having a predetermined length and activated with a positive voltage, is fed to the surface of the electrolyte 5 until the workpiece 3 comes into contact with the electrolyte 5 while measuring a contact point, at which an electric current initially flows into the electrolyte 5. This contact point measuring step S10 is the first step of the machining process of this invention.

[0032] The object of performing the contact point measuring step S10 is to measure the influence of the surface tension of the electrolyte 5 upon the workpiece 3 when the workpiece 3 acting as the anode is sunk into the electrolyte, and to allow the workpiece 3 to be more precisely machined in the machining step.

[0033] At the machining preparing step S20, the workpiece 3 is fed to the surface of the electrolyte 5. Thereafter, the applied voltage is removed from the workpiece 3 before the workpiece 3 is sunk into the electrolyte 5 by a length, which is predetermined on the basis of the contact point and to which the workpiece 3 has to be machined.

[0034] The initial value setting step S30 is performed to set the target length of the workpiece 3, the target diameter of the workpiece 3, the electrochemical equivalent volume constant of the workpiece 3, the current density, and the machining intervals.

[0035] At the machining step S40, the workpiece 3 is electrochemically machined in response to voltages applied to the workpiece 3 and the cathode rod 1. At this step S40, it is necessary to continuously calculate and measure a change in the surface area of the workpiece 3, the amount of applied current, the amount of electricity according to the applied current and the variable diameter of the workpiece 3 in accordance with the lapse in processing time during machining.

[0036] At the process-end determining step S50, it is determined whether the diameter of the machined workpiece 3 from the machining step S40 is equal to the target diameter, thus repeating the machining step S40 until the target diameter of the workpiece 3 is accomplished. Of course, the machining step S40 is ended when the target diameter of the workpiece 3 is accomplished. This process-end determining step S50 is the final step of the machining process of this invention.

[0037] In the electrochemical machining process of this invention comprising the above-mentioned five steps, both the amount of applied current and the current density are properly and steadily controlled by a computer in accordance with the physical and chemical properties of the workpiece 3, thus creating a diffusion effect capable of compensating for the conventional geometric effect. Such a diffusion effect thickens the tip of the cylindrical workpiece, thus preferably and effectively compensating for the conventional geometric effect sharpening the tip of the workpiece. Therefore, the electrochemical machining process of this invention produces a precise product having a uniform diameter along its length due to the preferred compensation of the diffusion effect for the conventional geometric effect. In order to achieve the above object, it is necessary to properly control both the amount of applied current and current density so as to maintain the metal dissolution rate of the workpiece 3 and the ion diffusion rate of the workpiece 3 during the electrochemical machining process.

[0038] In order to accomplish the precise machining results of the electrochemical machining process according to this invention, the workpiece 3 is ultrasonically washed on its surface with both acetone and distilled water before the contact point measuring step S10. It is thus possible to remove impurities from the surface of the workpiece 3.

[0039] The position of the cathode rod 1, the workpiece 3 and the electrolyte 5 during the electrochemical machining process of this invention is shown in FIG. 2.

[0040]FIG. 2 is a diagram, showing a system for performing the electrochemical machining process using the current density controlling techniques in accordance with the preferred embodiment of this invention.

[0041] As shown in FIG. 2, the electrochemical machining process of this invention is performed with the electrolyte 5, which is a potassium hydroxide solution having a mole number of 4˜6 mol and contained with a container having a predetermined size. Both the cathode rod 1 acting as the cathode and the workpiece 3 acting as the anode are sunk into the electrolyte 5, and are activated with electricity applied from a power source under the control of the computer. The excess metal of the workpiece 3 is thus electrochemically dissolved into the electrolyte 5, and so the workpiece 3 is machined to become the desired product.

[0042] During such an electrochemical machining process, the variable surface area of the workpiece 3, the amount of applied current, the amount of applied electricity, and the variable diameter of the workpiece 3 are calculated by the computer in response to input signals sent from a current detector. The calculated results are displayed on a display under the control of the computer. During the electrochemical machining process, the computer controls the power supply to apply an electric current to both the cathode rod 1 and the workpiece 3 while controlling the current until the workpiece 3 is machined to accomplish the target diameter.

[0043]FIG. 3 is a flowchart, showing in detail the flow of both the machining step S40 and the process-end determining step S50 included in the electrochemical machining process of FIG. 1.

[0044] As shown in FIG. 3, the machining step S40 is started after the target length, the target diameter, and the electrochemical equivalent volume constant of the workpiece 3, the current density, and the machining intervals are set in the initial value setting step S30.

[0045] At the machining step S40, both the cathode rod 1 acting as the cathode and the workpiece 3 acting as the anode, which are sunk into the electrolyte 5, are activated with electricity applied from the power source under the control of the computer. Therefore, the excess metal of the workpiece 3 is electrochemically dissolved into the electrolyte 5, thus being machined into a desired product. In such a case, the variable surface area of the workpiece 3, the amount of applied current, the amount of applied electricity, and the variable diameter of the workpiece 3 are continuously calculated and measured by the computer in accordance with the lapse in processing time during machining.

[0046] This machining step S40 is continuously repeated until the diameter of the machined workpiece 3 becomes the target diameter. When it is determined at the process-end determining step S50 that the diameter of the machined workpiece 3 from the machining step S40 becomes the target diameter, the machining step S40 is ended

[0047] In such a case, the variable surface area of the workpiece 3 during the machining step S40 is calculated by the expression A_(m)=α[LD+h(D_(o)+2D)/3], wherein A_(m) is the variable surface area (mm²) of the workpiece 3 during machining, L is the target length (mm) of the workpiece 3, h is the contact length (mm) of the workpiece 3 due to the surface tension, D is the variable diameter (mm) of the workpiece 3 during machining, and D_(o) is the original diameter (mm) of the workpiece 3.

[0048] In addition, the amount of applied current during the machining step S40 is calculated by the expression i=A_(m)J, wherein i is the current (C/sec) applied to the cathode rod and the workpiece during the unit of time (sec), A_(m) is the variable surface area (mm²) of the workpiece 3 during machining, and J is the current density (C/mm²sec).

[0049] On the other hand, the amount of electricity according to the applied current during the machining step S40 is calculated by the expression Q_(t)=Q_(p)+iΔt, wherein Q_(t) is the total amount of applied electricity (C) during machining, Q_(p) is the amount of electricity (C) applied during the previous step, and Δt is the variable machining time (sec).

[0050] The variable diameter of the workpiece 3 during the machining step S40 is calculated by the expression π(D_(o)−D)[L(D_(o)+D)/4+h(3D_(o)+2D)/15]α_(e)=Q_(t), wherein D is the variable diameter (mm) of the workpiece 3 during machining, D_(o) is the original diameter (mm) of the workpiece 3, Q_(t) is the total amount of applied electricity (C) during machining, L is the target length (mm) of the workpiece 3, h is the contact length (mm) of the workpiece 3 due to the surface tension, and α_(e) is the electrochemical equivalent volume constant (mm³/C) of the workpiece 3.

[0051]FIG. 4 is a flowchart, showing in detail the flow of the contact point measuring step S10 included in the electrochemical machining process of FIG. 1.0

[0052] As shown in FIG. 4, at the contact point measuring step S10, the cathode rod 1 activated with a negative voltage is primarily sunk into the electrolyte 5 within the container. On the other hand, the cylindrical workpiece 3 activated with a positive voltage is secondarily fed to the surface of the electrolyte 5 until the workpiece 3 initially comes into contact with the electrolyte 5 while measuring the contact point, at which an electric current initially flows into the electrolyte 5. That is, when the workpiece 3 activated with the positive voltage initially comes into contact with the electrolyte 5, an electric current initially flows in the electrolyte 5 due to the negative voltage applied to the cathode rod 1 sunk into the electrolyte 5. It is thus possible to precisely sense the current initially flowing in the electrolyte 5 and measure the desired contact point.

[0053] The object of performing the contact point measuring step S10 is to measure the influence of the surface tension of the electrolyte 5 upon the workpiece 3 during the machining step and to allow the workpiece 3 to be more precisely machined in the machining step. When the contact point is precisely measured, it is possible to calculate an additionally machined volume of metal of the workpiece 3 due to the surface tension of the electrolyte 5 upon the workpiece 3. The additionally machined volume of metal of the workpiece 3 due to the surface tension is calculated by the expression V_(p)=πh(−2D²−D_(o)D+3D₀ ²)/15, wherein V_(p) is the additionally machined volume (mm³) of metal of the workpiece 3 due to the surface tension, h is the contact length (mm) of the workpiece 3 due to the surface tension, D is the variable diameter (mm) of the workpiece 3 during machining, and D_(o) is the original diameter (mm) of the workpiece 3.

[0054] In the preferred embodiment of the present invention, the cathode rod 1 is a carbon rod, while the electrolyte 5 is a potassium hydroxide solution. However, it should be understood that the materials of both the cathode rod 1 and the electrolyte 5 may be freely changed without affecting the function of this invention. In addition, it is possible to machine desired products having a variety of shapes by properly changing the processing conditions, such as the amount of applied current, current density and mole number of electrolyte during the machining process.

[0055] As described above, the present invention provides an electrochemical machining process using current density controlling techniques. In the electrochemical machining process of this invention, it is possible to electrochemically machine a workpiece while properly controlling both the metal ion dissolving rate of the workpiece and the metal ion diffusing rate of the workpiece by controlling the amount of the applied current to make the two rates maintain a desired balance. This electrochemical machining process thus effectively produces a precise product having a uniform diameter along its length. In addition, when the electrochemical machining process is performed while properly changing the processing conditions, it is possible to produce a variety of products having different diameters. Another advantage of the electrochemical machining process of this invention resides in that the process is performed in consideration of an influence created by the surface tension of the electrolyte on a workpiece, and so it is possible to more precisely machine workpieces.

[0056] Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

What is claimed is:
 1. An electrochemical machining process using current density controlling techniques, comprising: a contact point measuring step of sinking a cathode rod activated with a negative voltage into an electrolyte within a container, and feeding a cylindrical workpiece having a predetermined length and activated with a positive voltage to a surface of said electrolyte until the workpiece comes into contact with the electrolyte while measuring a contact point, at which an electric current initially flows into the electrolyte; a machining preparing step of feeding the workpiece to the surface of the electrolyte and removing the applied voltage from the workpiece, and sinking the workpiece in the electrolyte by a length, which is predetermined on the basis of said contact point and to which the workpiece has to be machined; an initial value setting step of setting a target length of the workpiece, a target diameter of the workpiece, an electrochemical equivalent volume constant of the workpiece, a current density, and machining intervals; a machining step of applying voltages to both the workpiece and the cathode rod to electrochemically machine the workpiece while continuously calculating and measuring a variable surface area of the workpiece, the amount of applied current, the amount of electricity according to the applied current, and a variable diameter of the workpiece in accordance with the lapse in machining time; and a process-end determining step of determining whether the diameter of the machined workpiece from the machining step is equal to the target diameter, thus repeating the machining step until the target diameter of the workpiece is accomplished or stopping the machining step when the target diameter of the workpiece is accomplished.
 2. The electrochemical machining process according to claim 1, wherein said variable surface area of the workpiece during the machining step is calculated by the following expression A _(m) π[LD+h(D _(o)+2D)/3] wherein A_(m) is the variable surface area (mm²) of the workpiece during machining, L is a target length (mm) of the workpiece, h is a contact length (mm) of the workpiece due to surface tension, D is the variable diameter (mm) of the work-piece during machining, and D_(o) is an original diameter (mm) of the workpiece.
 3. The electrochemical machining process according to claim 1, wherein said amount of applied current during the machining step is calculated by the following expression i=A_(m)J wherein i is the applied current (C/sec) during a unit of time, A_(m) is the variable surface area (mm²) of the workpiece during machining, and J is the current density (C/mm²sec).
 4. The electrochemical machining process according to claim 1, wherein said amount of electricity during the machining step is calculated by the following expression Q _(t) =Q _(p) +iΔt wherein Q_(t) is the total amount of applied electricity (C) during machining, Q_(p) is the amount of electricity (C) applied during a previous step, and Δt is a variable machining time (sec).
 5. The electrochemical machining process according to claim 1, wherein said variable diameter of the workpiece during the machining step is calculated by the following expression π(D _(o) −D)[L(D _(o) +D)/4+h(3D _(o)+2D)/15]α_(e) =Q _(t) wherein D is the variable diameter (mm) of the workpiece during machining, D_(o) is an original diameter (mm) of the workpiece, Q_(t) is the total amount of applied electricity (C) during machining, L is a target length (mm) of the workpiece, h is a contact length (mm) of the workpiece due to surface tension, and α_(e) is the electrochemical equivalent volume constant (mm³/C) of the workpiece.
 6. The electrochemical machining process according to claim 1, wherein both a metal ion dissolving rate and a metal ion diffusing rate of the workpiece are controlled by controlling the amount of the applied current.
 7. The electrochemical machining process according to claim 1, wherein said cathode rod is a carbon rod.
 8. The electrochemical machining process according to claim 1, wherein said electrolyte is a potassium hydroxide solution.
 9. The electrochemical machining process according to claim 8, wherein said potassium hydroxide solution has a mole number of 4˜6 mol.
 10. The electrochemical machining process according to claim 1, wherein said workpiece is ultrasonically washed on its surface with both acetone and distilled water before the contact point measuring step so as to remove impurities from the surface of the workpiece.
 11. The electrochemical machining process according to claim 1, wherein an additionally machined volume of metal of the workpiece due to surface tension is calculated by the following expression V _(p) =πh(−2D ² −D _(o) D+3D ₀ ²)/15 wherein V_(p) is the additionally machined volume (mm³) of metal of the workpiece due to the surface tension, h is a contact length (mm) of the workpiece due to the surface tension, D is the variable diameter (mm) of the workpiece during machining, and D^(o) is an original diameter (mm) of the workpiece. 